The most commonly used system for position detection is based on a mobile receiver device receiving time signals transmitted by radio along a line of sight from satellites. The system allows the mobile receiver determining its location in terms of longitude, latitude, and altitude/elevation to high precision, for instance within a few meters. The system can be used for providing position, navigation or for tracking the position of something fitted with a receiver. Examples of such kind of satellite navigation systems with global coverage are the US American Global Positioning System (GPS) and the European Galileo system. For the sake of brevity, in the following, reference will only be made to GPS as a representative for all other kinds of satellite navigation systems.
Today, there is a plethora of location-based services that require knowing the position of the user of a service. However, for cost or other reasons a GPS system is not always available. A very cost effective approach to implement location-based services is if location determination is performed by utilizing hardware that is already installed for other purposes like wireless data connection. For example, in an agricultural environment there are many monitoring and measuring devices installed, which are dedicated to measure environmental parameters to determine for instance a need for irrigation or fertilization in certain areas of a field.
Specifically, it is possible to utilize known positions of existing radio base stations that can be detected from a user's computational device (e.g., laptop or tablet computer, PDA, smart phone, wristwatch, etc.). One example is Wi-Fi (802.11) radio networking. By detecting one or more Wi-Fi base stations (“access points”), a device can compute its own latitude, longitude, and/or elevation based on knowledge of the base stations' positions. Other radio base stations (e.g., cell phone towers, AM and FM radio stations, TV stations, etc.) can be utilized in a similar manner. The base stations can be considered as radio beacons.
Utilizing existing radio base stations means a user's computational device can compute its own location without a GPS receiver. Utilizing an existing radio receiver that is already part of the user's computational device for data transmission reduces cost while availability of the service is increased. Furthermore, since base stations are usually detectable in situations where GPS fails, this technique works where GPS does not.
Generally, in location detection systems using terrestrial transmitters a pseudorandom signal is emitted by the transmitters having a known location and the signal is received by a mobile receiver. The transmitter transmits a characteristic binary signal that is known and detected by the receiver with a matched filter in order to determine from the filter response a precise point of time when the signal is received. Ranging (i.e. distance estimation) using radio frequency (RF) waveforms is performed by estimating the time-of-arrival (ToA) of a received waveform at the receiver, being transmitted by the transmitter. In that way the time-of-arrival (ToA) can be determined. By sampling this waveform with a low sample rate, precise estimation of the ToA is challenging and leads most of the times to reduced ranging (and positioning) precision. This effect of reducing the ranging precision due to finite sample rate is called “range binning”. In other words, the accuracy of the location determination depends on the granularity of quantization in the time domain to determine an accurate ToA. However, conventional Wi-Fi, or other commonly used data transmission hardware does not always provide for a high sample rate enabling a location determination that is sufficiently precise for most location based services.
Reducing the range binning effect, i.e. increasing the ranging precision, is normally performed by increasing the sample rate of the A/D converters in the receivers. This is usually an expensive solution, since the price of the A/D converters depends on their sample rate.
The most common method for increasing the sample rate, without using a high sample rate A/D converter is called equivalent time sampling (ETS). However, the ETS approach is performed and is applicable only on periodic waveforms and is mainly used in oscilloscopes. Furthermore, the ETS approach requires A/D converters having the capability to provide subsample delays.
In RF ranging and positioning applications this method usually cannot be implemented due to the lack of A/D converters which can perform subsample delays. The RF ranging and positioning methods are commonly implemented on standard RF data transceivers, which are only capable of equidistant sample acquisition and do not have any capability for synchronization with the incoming waveform or performing subsample delays.
For alleviating this problem a method for a modified equivalent time sampling (METS) was proposed. Using this method, the periodic waveform is prepared at the transmitter. The subsample delays are introduced in the transmitted signal between each two copies of the waveform to be oversampled. The receiver samples the incoming waveform equidistantly and stores the samples in memory. The stored samples are later interleaved in order to reconstruct an oversampled version of the waveform. The METS is designed to support a fixed oversampling rate. In systems with multiple receivers among which not all have the capacity to handle the fixed oversampling rate this presents a problem.
Thus, there remains a need for an alternative positioning system which is miniaturized and less costly than a GPS system and works even when there is no reception of a sufficient number of satellites to perform the positioning of the receiver.